1. Field of the Invention
This invention relates to a shock sound prevention circuit used in a BTL (Balanced Transformer-less) audio amplifier, specifically to a shock sound prevention circuit for an audio amplifier using two operational amplifiers which can be biased with the same reference voltage.
2. Description of the Related Art
Some of electronic cameras such as video camcorders and digital still cameras are equipped with liquid crystal display monitors. When equipped with the liquid crystal display monitors, they need audio functions and require speaker functions. For example, a speaker function tells a user with an audio sound that a power is turned on, when the user turns the power on.
Such a sound is called a beep sound, and is generated to last about 100 ms after turning the power on. Many of the electronic cameras use low power supply voltage. Thus, they often use BTL method, which can secure wide dynamic range even at the low power supply voltage. The BTL method provides each of both ends of the speaker with each of a pair of signals having a phase opposite to each other, making it possible to secure the wide dynamic range.
An audio amplifier according to the BTL method is shown in FIG. 2. An audio signal is applied to an input terminal 1, as shown in FIG. 2. A terminal 2 and a terminal 3 are pins of an IC (integrated circuit). A capacitance C2 is an external part of the IC. The capacitance C2 is a blocking capacitor. The audio signal is once outputted from the IC, and then returned into the IC after DC (direct current) component is removed.
The audio signal returned into the IC is amplified by a first operational amplifier 4. A gain of the first operational amplifier 4 is determined according to a feedback ratio determined by a ratio between a resistance R9and a resistance R10. The audio signal is applied to a negative input terminal (−) of the first operational amplifier 4, thus a reverse audio signal is obtained at an output terminal 5. A capacitance C1 for reference voltage is charged from the IC upon turning-on of a power supply. A reference voltage is generated from a reference voltage generation circuit 6, as the capacitance C1 is charged. The reference voltage is applied as a bias voltage to positive input terminals (+) of the first operational amplifier 4 and a second operational amplifier 7.
The audio signal generated at the output terminal 5 of the first operational amplifier 4 is applied to a negative input terminal of the second operational amplifier 7. Thus, an audio signal with a phase reversed once again is obtained at an output terminal 8. In other words, an audio signal returned to a normal phase is obtained. The second operational amplifier operates similar to the first operational amplifier 4, and its gain is determined by a ratio between a resistance R11 and a resistance R12.
Therefore, a normal audio signal is obtained at the terminal 10 while the reverse audio signal is obtained at the terminal 9. A speaker 11 is driven by the two audio signals to make a sound such as the beep sound.
However, an audio amplifier usually generates a shock sound when the power supply is turned on. On that account, there arises a problem that the shock sound disturbs the beep sound.
With this being the situation, this invention offers a shock sound prevention circuit in the BTL audio amplifier as shown in FIG. 2, which enables a stable operation 100 msec after turning-on of the power supply. Usually the audio amplifier shown in FIG. 2 is integrated in an IC. Use of capacitance, which takes up a large pattern area on the IC chip, can be minimized with this invention.